Fine structure of the longitudinal flagellum in Ceratium tripos, a marine dinoflagellate

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Abstract
The fine structure of the longitudinal flagellum in Ceratium tripos, which performs not only undulations but also retractions, has been examined in both the retracted and relaxed states. Although conventional fixation always triggered retraction, the flagellum was found to remain relaxed when it was washed briefly with Ca2+-free medium prior to fixation. Previous light-microscopic investigation showed that it contained two fibres, the axoneme and the R-fibre. The present study by transmission electron microscopy has revealed that the axoneme that appeared to be a single fibre under the light microscope is a bundle of four fibres; the 9 + 2 microtubular axoneme, the packing material, the striated fibre, and the paraxial fibre. The first two are common in the longitudinal flagella of dinoflagellates, and the axoneme presumably generates the undulation. The last two are new and unique to the longitudinal flagellum of Ceratittm. The R-fibre, which probably contracts to fold the flagellum during retraction, consists of fine filaments, which pursue a loosely spiral course in the contracted state, but align longitudinally in the relaxed state. Periodic striations appear only on the extended R-fibre. The R-fibre shortens to approximately one third of its extended length and pursues a left-handed helix. The packing material, which sticks to the microtubular axoneme on one side, is connected with the R-fibre on the other side at intervals of approximately 5–8 μm. The retraction seems to be regulated by the Ca2+ concentration in the flagellum.